Stacked flat panel x-ray detector assembly and method of making same

ABSTRACT

An x-ray detector assembly includes a first flat panel digital projection detector and a second flat panel digital projection detector. The x-ray detector assembly further includes a first detector mounting structure configured to align the first flat panel digital projection detector in a first position to block the second flat panel digital projection detector from receiving x-rays emitting from an x-ray source toward the second flat panel digital projection detector in an x-ray penetration direction.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of, and claims priority to, U.S. non-provisional application serial number 11/523,359, filed Sep. 19, 2006, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to diagnostic imaging methods and apparatus, and more particularly, to an apparatus and method of manufacturing a stacked flat panel x-ray detector assembly.

Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. Generally, the x-ray source and the detector assembly are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors.

The detector assembly is typically made of a plurality of detector modules. Data representing the intensity of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis that ultimately produces an image.

Conventional CT systems emit an x-ray with a polychromatic spectrum. The x-ray attenuation of each material in the subject depends on the energy of the emitted x-ray. If CT projection data is acquired at multiple x-ray energy levels or spectra, the data contains additional information about the subject or object being imaged that is not contained within a conventional CT image. For example, spectral CT data can be used to produce a new image with x-ray attenuation coefficients equivalent to a chosen monochromatic energy. Such a monochromatic image includes image data where the intensity values of the voxels are assigned as if a CT image were created by collecting projection data from the subject with a monochromatic x-ray beam. Spectral CT data facilitates better discrimination of tissues, making it easier to differentiate between materials such as tissues containing calcium and iodine, for example.

A principle objective of energy sensitive scanning is to obtain diagnostic CT images that enhance information (contrast separation, material specificity, etc.) within the image by utilizing two or more scans at different chromatic energy states. High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two or more energy sensitive scans may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as typically occurs with previous CT technology. The interleaved projection data may furthermore be registered so that the same path lengths are defined at each energy level using, for example, some form of interpolation.

Conventional curvilinear detector array include a large number of individual detector elements arranged on the detector array. The detector elements are scintillator/photodiode cells arranged in two-dimensional modules that are then combined into two-dimensional detector area arrays. The coverage area of the detector array is defined by the number of detector elements in each 2D module and the number of 2D modules that are combined to form the detector assembly.

While known systems and methods that include conventional curvilinear detector arrays can be employed to acquire projection data at multiple x-ray energy levels or spectra and display, the coverage area of a scan is defined by the size of the detector array. Because each individual detector element has its own element-specific readout channel, the larger the detector assembly the more costly and complex the imaging system becomes.

For various imaging applications, such as cardiac scanning, it would be advantageous to acquire all of the image data for the object being imaged in a single rotation of the gantry. Such an image data acquisition technique has a number of benefits, including minimizing motion artifacts, as an example. However, the coverage area of the detector assembly must be sized based on the size of the projection of the object being imaged on the detector array. A curvilinear detector assembly designed with a large enough coverage area to image a heart, for example, would be extremely complex and cost prohibitive.

Therefore, it would be desirable to design an x-ray detector assembly that overcomes the aforementioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, an x-ray detector assembly includes a first flat panel digital projection detector and a second flat panel digital projection detector. The x-ray detector assembly further includes a first detector mounting structure configured to align the first flat panel digital projection detector in a first position to block the second flat panel digital projection detector from receiving x-rays emitting from an x-ray source toward the second flat panel digital projection detector in an x-ray penetration direction.

In accordance with another aspect of the invention, a method of manufacturing a detector assembly includes the step of providing a first flat panel detector comprising a top surface configured to face an x-ray source, the first flat panel detector having a width defined in a slice direction between a first side and a second side of the first flat panel detector and a length defined in a channel direction between a third side and a fourth side of the first flat panel detector. The method also includes the step of providing a second flat panel detector comprising a top surface configured to face the x-ray source, the second flat panel detector having a width defined in a slice direction between a first side and a second side of the second flat panel detector and a length defined in a channel direction between a third side and a fourth side of the second flat panel detector. Further, the method includes the step of coupling the first flat panel detector to a mounting assembly having an obstructing position configured to align the first flat panel detector above the top surface of the second flat panel detector in an x-ray penetration direction such that the first flat panel detector is substantially aligned with the second flat panel detector in the slice and channel directions.

In accordance with another aspect of the invention, a CT system includes a rotatable gantry having an opening therein for receiving an object to be scanned, a table positioned within the opening of the rotatable gantry and moveable through the opening in a z-direction, and an x-ray source coupled to the rotatable gantry and configured to project a beam of x-rays toward the object to be scanned. The CT system also includes a detector assembly positioned to receive the beam of x-rays from the x-ray source. The detector assembly includes a first flat panel digital detector and a second flat panel digital detector. The first flat panel digital detector is arranged between the second flat panel digital detector and the x-ray source such that a first plurality of x-rays projecting from the x-ray source toward the second flat panel digital detector are absorbed by the first flat panel digital detector.

Various other features and advantages will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated for carrying out the invention.

In the drawings:

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a perspective view of one embodiment of a CT system detector array.

FIG. 4 is a perspective view of one embodiment of a detector.

FIG. 5 is a cross-sectional view of a portion of an imaging system with a detector assembly in accordance with an embodiment of the invention.

FIG. 6 is a perspective view of the detector assembly of FIG. 5 in accordance with an embodiment of the invention.

FIG. 7 is a block schematic diagram of the detector assembly of FIG. 5 illustrating a flat panel detector in a first position.

FIG. 8 is another block schematic diagram of the detector assembly of FIG. 5 illustrating the flat panel detector in a second position.

FIG. 9 is a cross-sectional view of a portion of an imaging system with a detector assembly in accordance with an embodiment of the invention.

FIG. 10 is a pictorial view of a CT system for use with a non-invasive package inspection system.

DETAILED DESCRIPTION

The operating environment of the invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the invention is equally applicable for use with other multi- slice configurations. Moreover, the invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems.

In addition, certain embodiments of the present invention provide systems, methods, and computer instructions for acquiring multi-energy data, such as dual energy data, for example. Certain multi-energy data can be used in spectral imaging systems, such as photon counting systems, for example. Dual energy data, which is a type of multi-energy data, can be embodied in monochromatic images, material density images, and/or effective-Z images. While many of the embodiments described herein are discussed in connection with dual energy data, the embodiments are not limited to dual energy data and can be used in connection with other types of multi-energy data, as one skilled in the art will appreciate.

Referring to FIG. 1, a CT imaging system 10 is shown as including a gantry 12 representative of a “third generation” CT scanner. Gantry 12 has an x-ray source 14 that projects a beam of x-rays toward a detector assembly or collimator 16 on the opposite side of the gantry 12. Referring now to FIG. 2, detector assembly 16 is formed by a plurality of detectors or detector modules 18 and data acquisition systems (DAS) 20. The plurality of detectors 18 sense the projected x-rays 22 that pass through a medical patient 24, and DAS 20 converts the data to digital signals for subsequent processing. Each detector 18 produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient 24. During a scan to acquire x-ray projection data, gantry 12 and the components mounted thereon rotate about a center of rotation 26.

Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 28 of CT system 10. Control mechanism 28 includes an x-ray controller 30 that provides power and timing signals to an x-ray source 14 and a gantry motor controller 32 that controls the rotational speed and position of gantry 12. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 20 and performs high speed reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.

Computer 36 also receives commands and scanning parameters from an operator via console 40 that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 20, x-ray controller 30 and gantry motor controller 32. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 24 and gantry 12. Particularly, table 46 moves patients 24 through a gantry opening 48 of FIG. 1 in whole or in part.

As shown in FIG. 3, detector assembly 16 includes rails 50 having collimating blades or plates 52 placed therebetween. Plates 52 are positioned to collimate x-rays 22 before such beams impinge upon, for instance, detector 18 of FIG. 4 positioned on detector assembly 16. In one embodiment, detector assembly 16 includes 57 detectors 18, each detector 18 having an array size of 64×16 of pixel elements 54. As a result, detector assembly 16 has 64 rows and 912 columns (16×57 detectors) which allows 64 simultaneous slices of data to be collected with each rotation of gantry 12.

Referring to FIG. 4, detector 18 includes DAS 20, with each detector 18 including a number of detector elements 54 arranged in pack 56. Detectors 18 include pins 58 positioned within pack 56 relative to detector elements 54. Pack 56 is positioned on a backlit diode array 60 having a plurality of diodes 62. Backlit diode array 60 is in turn positioned on multi-layer substrate 64. Spacers 66 are positioned on multi-layer substrate 64. Detector elements 54 are optically coupled to backlit diode array 60, and backlit diode array 60 is in turn electrically coupled to multi-layer substrate 64. Flex circuits 68 are attached to face 70 of multi-layer substrate 64 and to DAS 20. Detectors 18 are positioned within detector assembly 16 by use of pins 58.

In the operation of one embodiment, x-rays impinging within detector elements 54 generate photons which traverse pack 56, thereby generating an analog signal which is detected on a diode within backlit diode array 60. The analog signal generated is carried through multi-layer substrate 64, through flex circuits 68, to DAS 20 wherein the analog signal is converted to a digital signal.

FIG. 5 illustrates a side view diagram of a portion of an imaging system 72, such as, for example, imaging system 26 of FIG. 1. Imaging system 72 includes an x-ray source 74 that is configured to rotate about a center of rotation 76 of a rotatable gantry 78. A beam of x-rays 80 is produced when high-speed electrons emitted from x-ray source 74 impact the surface of a target portion (not shown) of x-ray source 74. Beam of x-rays 80 passes through a patient 82 and impinges upon a detector assembly 84.

Referring now to FIGS. 5 and 6, detector assembly 84 includes a first flat panel digital projection radiographic detector 86 coupled to a first mounting bracket 88 and a second flat panel digital projection radiographic detector 90 coupled to a second mounting bracket 92. Flat panel detectors 86, 90 are solid state digital projection radiographic detectors such as digital fluoroscopy panels typically used for catheter imaging. Each flat panel detector 86, 90 includes an array of pixels 94 arranged in rows and columns. Unlike a conventional curvilinear detector assembly that is typically used in CT imaging systems and that has an individual readout channel for each detector module of the curvilinear detector assembly, such as, for example, detector assembly 16 (FIG. 3), pixels 94 of flat panel detectors 86, 90 are associated with a matrix of scan lines 96 and read lines 98, as shown in FIG. 6. Each flat panel detector 86, 90 includes a single readout channel 100 per read line 98. In one embodiment, flat panel detectors 86, 90 are Revolution™ XR/d detectors commercially available from the GE Healthcare business of the General Electric Company.

Flat panel detectors 86, 90 may be constructed having different attenuation characteristics, according to various embodiments. For example, the scintillators of flat panel detectors 86, 90 may have different thicknesses (measured in the y-direction 102) or be made with different scintillator materials such that first flat panel detector 86 absorbs lower-energy x-rays and second flat panel detector 90 absorbs higher-energy x-rays.

Detector assembly 84 is aligned with x-ray source 74 such that beam of x-rays 80 passes through patient 82 and initially impinges upon first flat panel detector 86, which is aligned with a region of interest 104 of patient 82. As shown in FIG. 5, first flat panel detector 86 is positioned between x-ray source 74 and second flat panel detector 90 in an x-ray penetration direction 106. Thus, after passing through patient 82, beam of x-rays 80 impinges upon a top surface 108 of first flat panel detector 86. A portion of beam of x-rays 80 is absorbed by first flat panel detector 86 and a portion of beam of x-rays 80 pass through first flat panel detector 86 and impinges upon a top surface 110 of second flat panel detector 90. First flat panel detector 86 has a width measured in the x-direction 112 (channel direction) between a first side 114, and second side 116, and a length measured in the z-direction 118 (slice direction) between a third side 120 and a fourth side 122. Likewise, second flat panel detector 90 has a width measured in the x-direction 112 between respective first and second sides 124,126 and a length measured in the z-direction 118 between respective third and fourth sides 128, 130.

The coverage area of first flat panel detector 86 is at least equal to the size of a full projection 132 of region of interest 104 on top surface 108 of first flat panel detector 86. Thus, first flat panel detector 86 may be the size of a projected organ of interest, such as a heart, liver, or lung according to various embodiments. In one embodiment, first flat panel detector 86 has a coverage area of approximately 20 cm in the x-direction 112 by 20 cm measured in the z-direction 118.

According to various embodiments, region of interest 104 corresponds to an organ being imaged, such as a heart, lung, or liver as examples. Thus, for a cardiac scanning application where the region of interest 104 is defined as the heart, a whole heart of an adult may be scanned during a single rotation of rotatable gantry 78, since the coverage area of first flat panel detector 86 is larger than the projection 132 of the heart on first flat panel detector 86.

Optionally, flat panel collimator assemblies or flat panel collimator assemblies or grids 134, 136 (shown in phantom) may be positioned in front of respective first and second flat panel detectors 86, 90 in the x-ray penetration direction 106 for collimating x-ray beams received at first and second flat panel detectors 86, 90.

In one embodiment, first mounting assembly or bracket 88 is a fixed bracket mounted within rotatable gantry 78 such that first flat panel detector 86 is substantially centered above second flat panel detector 90. Alternatively, first mounting bracket 88 comprises a pair of moveable guide rails that allow first flat panel detector 86 to be moved into and out of alignment with second flat panel detector 90 and into and out of beam of x-rays 80, as described in detail with respect to FIGS. 7 and 8.

FIG. 7 is a block schematic of first flat panel detector 86 of diagram detector assembly 84 in a first position, wherein first flat panel detector 86 is positioned above and substantially overlaps second flat panel detector 90. When first flat panel detector 86 is in the first position, a portion of x-rays 80 emitting toward detector assembly 84 in the x-ray penetration direction 104 are absorbed by first flat panel detector 86, while another portion of x-rays 82 pass through first flat panel detector 86 and impinge second flat panel detector 90.

FIG. 8 is a block schematic diagram of first flat panel detector 86 in a second position, wherein first flat panel detector 86 is offset from second flat panel detector 90 in the z-direction 118. Thus, when first flat panel detector 86 is in the second position, x-rays 80 directed toward detector assembly 84 in the x-ray penetration direction 104 impinge second flat panel detector 90 without first passing through first flat panel detector 86.

Referring now to FIG. 9, an imaging system 140 is shown according to an alternative embodiment. Imaging system 140 is configured in a similar manner as described with respect to imaging system 72 of FIG. 5 in that imaging system 140 includes x-ray source 74 positioned to direct a beam of x-rays 80 toward detector assembly 84, which includes a first flat panel detector 86 positioned above a second flat panel detector 90. In addition to the components included in imaging system 72 (FIG. 5), imaging system 140 includes a notch filter 142 positioned between the first and second flat panel detectors 86, 90 in the x-ray penetration direction 104 for use in energy discrimination computed tomography (EDCT) applications. Notch filter 142 is constructed of an x-ray attenuating material that provides for a greater energy separation band or notch between the higher energy x-rays and the lower energy x-rays in the band of x-rays 80 emitted from x-ray source 74. According to various embodiments, notch filter 142 may be constructed of a single material or a composite material to broaden the width of the notch of filtered x-rays. Further, imaging system 140 may include a number of interchangeable notch filters 142 having different filtering properties, which can be selected based on the specifications of a given scan. Notch filter 142 may be rigidly or slidably mounted on guide rails 144, in a similar manner as described above with respect to first flat panel detector 86. One skilled in the art will recognize that an imaging system may include a pair of flat panel detectors, similar to detectors 86, 90, without a notch filter, according to an alternative embodiment. Further, notch filter 142 may be positioned between patient 82 and first flat panel detector 86.

Imaging system 140 may also include a collimating assembly or grid (not shown), similar to optional collimating assembly flat panel collimator assemblies or grids 134, 136 (FIG. 5), positioned in front of each flat panel detector 86, 90.

By combining two flat panel detectors 86, 90, the resulting detector assembly 84 mitigates the negative aspects inherent in the use of flat panel detectors for CT imaging applications. For example, a detector assembly that includes two stacked detectors with different attenuation characteristics has an improved dynamic range over a single flat panel detector. Further, the use of flat panel detector technology in a CT imaging system has a number of advantages over of a conventional curvilinear detector assembly. Because the manufacture of a flat panel detector is more cost effective than a curvilinear detector having a similar coverage area, use of a flat panel detector reduces the overall cost of detector assembly while allowing for increased coverage in the z-direction 118 (i.e., along the patient axis). Thus, detector assembly 84 is particularly advantageous for use in cardiac CT imaging applications, as flat panel detectors 86, 90 provide the resolution and coverage desired for cardiac imaging.

Referring now to FIG. 10, a package/baggage inspection system 146 includes a rotatable gantry 148 having an opening 150 therein through which packages or pieces of baggage may pass. The rotatable gantry 148 houses a high frequency electromagnetic energy source 152 as well as a detector assembly or collimator 154 having scintillator arrays comprised of scintillator cells similar to that shown in FIG. 3 or 4. A conveyor system 156 is also provided and includes a conveyor belt 158 supported by structure 160 to automatically and continuously pass packages or baggage pieces 162 through opening 150 to be scanned. Objects 162 are fed through opening 150 by conveyor belt 158, imaging data is then acquired, and the conveyor belt 158 removes the packages 162 from opening 150 in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages 162 for explosives, knives, guns, contraband, etc.

One skilled in the art will appreciate that embodiments of the invention may be interfaced to and controlled by a computer readable storage medium having stored thereon a computer program. The computer readable storage medium includes a plurality of components such as one or more of electronic components, hardware components, and/or computer software components. These components may include one or more computer readable storage media that generally stores instructions such as software, firmware and/or assembly language for performing one or more portions of one or more implementations or embodiments of a sequence. These computer readable storage media are generally non-transitory and/or tangible. Examples of such a computer readable storage medium include a recordable data storage medium of a computer and/or storage device. The computer readable storage media may employ, for example, one or more of a magnetic, electrical, optical, biological, and/or atomic data storage medium. Further, such media may take the form of, for example, floppy disks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/or electronic memory. Other forms of non-transitory and/or tangible computer readable storage media not list may be employed with embodiments of the invention.

A number of such components can be combined or divided in an implementation of a system. Further, such components may include a set and/or series of computer instructions written in or implemented with any of a number of programming languages, as will be appreciated by those skilled in the art. In addition, other forms of computer readable media such as a carrier wave may be employed to embody a computer data signal representing a sequence of instructions that when executed by one or more computers causes the one or more computers to perform one or more portions of one or more implementations or embodiments of a sequence.

Therefore, in accordance with one embodiment, an x-ray detector assembly includes a first flat panel digital projection detector and a second flat panel digital projection detector. The x-ray detector assembly further includes a first detector mounting structure configured to align the first flat panel digital projection detector in a first position to block the second flat panel digital projection detector from receiving x-rays emitting from an x-ray source toward the second flat panel digital projection detector in an x-ray penetration direction.

In accordance with another embodiment, a method of manufacturing a detector assembly includes the step of providing a first flat panel detector comprising a top surface configured to face an x-ray source, the first flat panel detector having a width defined in a slice direction between a first side and a second side of the first flat panel detector and a length defined in a channel direction between a third side and a fourth side of the first flat panel detector. The method also includes the step of providing a second flat panel detector comprising a top surface configured to face the x-ray source, the second flat panel detector having a width defined in a slice direction between a first side and a second side of the second flat panel detector and a length defined in a channel direction between a third side and a fourth side of the second flat panel detector. Further, the method includes the step of coupling the first flat panel detector to a mounting assembly having an obstructing position configured to align the first flat panel detector above the top surface of the second flat panel detector in an x-ray penetration direction such that the first flat panel detector is substantially aligned with the second flat panel detector in the slice and channel directions.

In accordance with yet another embodiment, a CT system includes a rotatable gantry having an opening therein for receiving an object to be scanned, a table positioned within the opening of the rotatable gantry and moveable through the opening in a z-direction, and an x-ray source coupled to the rotatable gantry and configured to project a beam of x-rays toward the object to be scanned. The CT system also includes a detector assembly positioned to receive the beam of x-rays from the x-ray source. The detector assembly includes a first flat panel digital detector and a second flat panel digital detector. The first flat panel digital detector is arranged between the second flat panel digital detector and the x-ray source such that a first plurality of x-rays projecting from the x-ray source toward the second flat panel digital detector are absorbed by the first flat panel digital detector.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

1. An x-ray detector assembly comprising: a first flat panel digital projection detector; a second flat panel digital projection detector; and a first detector mounting structure configured to align the first flat panel digital projection detector in a first position to block the second flat panel digital projection detector from receiving x-rays emitting from an x-ray source toward the second flat panel digital projection detector in an x-ray penetration direction.
 2. The x-ray detector assembly of claim 1 wherein the first and second flat panel digital projection detectors comprise digital projection radiographic detectors having a coverage area at least equal in size to a complete projection of an average adult heart.
 3. The x-ray detector assembly of claim 1 wherein the first detector mounting structure comprises guide rails configured to allow the first flat panel digital projection detector to be moved between the first position and a second position; and wherein, when in the second position, the first flat panel digital projection detector is misaligned with the second flat panel digital projection detector such that x-rays emitting from the x-ray source in the x-ray penetration direction impinge upon the second flat panel digital projection detector without passing through the first flat panel digital projection detector.
 4. The x-ray detector assembly of claim 1 further comprising a notch filter comprising an x-ray attenuating material selected to filter x-rays within a given energy band.
 5. The x-ray detector assembly of claim 4 wherein the notch filter is positioned between the first and second flat panel digital projection detectors.
 6. The x-ray detector assembly of claim 4 wherein the notch filter is positioned between the first panel digital projection detector and the x-ray source.
 7. The x-ray detector assembly of claim 1 wherein the first flat panel digital projection detector is configured to absorb low-energy x-rays; and wherein the second flat panel digital projection detector is configured to absorb high-energy x-rays.
 8. The x-ray detector assembly of claim 7 wherein the first flat panel digital projection detector has a thickness in the x-ray penetration direction that is less than a thickness of the second flat panel digital projection detector.
 9. The x-ray detector assembly of claim 1 wherein the first and second flat panel digital projection detectors have a width in a channel direction of approximately 20 cm.
 10. A method of manufacturing a detector assembly comprising: providing a first flat panel detector comprising a top surface configured to face an x-ray source, the first flat panel detector having a width defined in a slice direction between a first side and a second side of the first flat panel detector and a length defined in a channel direction between a third side and a fourth side of the first flat panel detector; providing a second flat panel detector comprising a top surface configured to face the x-ray source, the second flat panel detector having a width defined in a slice direction between a first side and a second side of the second flat panel detector and a length defined in a channel direction between a third side and a fourth side of the second flat panel detector; and coupling the first flat panel detector to a mounting assembly having an obstructing position configured to align the first flat panel detector above the top surface of the second flat panel detector in an x-ray penetration direction such that the first flat panel detector is substantially aligned with the second flat panel detector in the slice and channel directions.
 11. The method of claim 10 wherein the mounting assembly is configured to align the first and second flat panel detectors such that x-rays emitting from an x-ray source and passing through the first flat panel detector in the x-ray penetration direction impinge upon the second flat panel detector.
 12. The method of claim 10 further comprising providing the mounting assembly having the obstructing position and a non-obstructing position, the non-obstructing position configured to misalign the first flat panel detector with respect to the second flat panel detector such that x-rays emitting from the x-ray source in the x-ray penetration direction impinge upon the second flat panel detector without passing through the first flat panel detector.
 13. The method of claim 10 further comprising positioning a notch filter above the top surface of the second flat panel detector in the x-ray penetration direction.
 14. A computed tomography (CT) system comprising: a rotatable gantry having an opening therein for receiving an object to be scanned; a table positioned within the opening of the rotatable gantry and moveable through the opening in a z-direction; an x-ray source coupled to the rotatable gantry and configured to project a beam of x-rays toward the object to be scanned; and a detector assembly positioned to receive the beam of x-rays from the x-ray source, the detector assembly comprising: a first flat panel digital detector; a second flat panel digital detector; and wherein the first flat panel digital detector is arranged between the second flat panel digital detector and the x-ray source such that a first plurality of x-rays projecting from the x-ray source toward the second flat panel digital detector are absorbed by the first flat panel digital detector.
 15. The CT system of claim 14 further comprising a notch filter positioned between the first flat panel digital detector and the second flat panel digital detector such that x-rays passing through the first flat panel digital projection detector pass through the notch filter prior to impinging upon the second flat panel digital detector.
 16. The CT system of claim 14 further comprising a notch filter positioned between the first flat panel digital detector and the x-ray source such that x-rays passing from the x-ray source pass through the notch filter prior to impinging upon the first flat panel digital detector.
 17. The CT system of claim 14 wherein the first flat panel digital detector has a thickness in an x-ray penetration direction that is less than a corresponding thickness of the second flat panel digital detector.
 18. The CT system of claim 14 wherein the first flat panel digital detector comprises a first scintillating material configured to absorb low-energy x-rays; and wherein the second flat panel digital detector comprises a second scintillating material configured to absorb high-energy x-rays.
 19. The CT system of claim 14 wherein the first flat panel digital detector has a coverage area that is large enough to acquire projection data for a whole adult heart during one rotation of the rotatable gantry.
 20. The CT system of claim 14 further comprising a data acquisition system configured to receive projection data from the first and second flat panel digital detectors. 